AMS’ most recent publication evaluates the importance of light versus nutrients in driving phytoplankton growth in San Francisco Bay which has an excess of nutrients relative to the requirement for phytoplankton growth. As nutrients have not traditionally been considered as important as light for regulating phytoplankton biomass in San Francisco Bay, we investigated the role different nitrogen sources at non-limiting conditions play in growth rates. We demonstrate that at intermediate light levels, growth rates of phytoplankton can be significantly higher when growing on ammonium-nitrogen compared with nitrate-nitrogen, even at non-limiting nutrient conditions. This nitrogen growth rate difference (ammonium growth/nitrate growth) disappears at low and high light conditions. Our study found this nitrogen growth difference to be notable in pelagic as well as in benthic diatoms isolated from northern San Francisco Bay, suggesting it affects phytoplankton populations commonly found at intermediate light levels throughout the bay.